1. Field of the Invention
The present invention relates to an SPM cantilever for use in a scanning probe microscope such as an atomic force microscope, and a method for manufacturing the SPM cantilever.
2. Description of the Related Art
Since a scanning tunneling microscope (STM) was invented by Binning, Rohrer, et al., as an apparatus for observing a conductive sample in a resolution on the order of the atomic size, it has been utilized in various fields as a microscope with which surface ruggedness can be observed in the atomic order. However, the sample, which can be observed by the STM, is limited to a conductive substance.
An atomic force microscope (AFM) has been proposed as a microscope to observe an insulating sample, which cannot be easily inspected by the STM, utilizing the element techniques, such as the servo technique in the STM. The AFM is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 62-130302.
The AFM is similar to the STM in structure, and classified as one of scanning probe microscopes (SPMs). In the AFM, a cantilever having a sharp projection (probe) at the free end is arranged near a measurement surface of a sample so as to be faced thereto. A movement of the cantilever, due to interaction between an atom at the end of the probe and an atom of the sample, is measured electrically or optically, while the sample or the probe is scanned in XY directions to relatively change the positional relationship between the cantilever and the probe. In this manner, information on the sample, such as the ruggedness, can be inspected three-dimensionally on the order of the atomic size.
A cantilever chip for use in the SPM, such as the AFM as described above, has been able to be produced in a high reproducibility with a high accuracy in the micron order, since T. R. Albrecht et al. proposed a cantilever made of silicon oxide film, which can be manufactured by the semiconductor IC manufacturing process (Thomas R. Albrecht and Calvin F. Quate: Atomic Resolution Imaging of a Nonconductor Atomforce Microscopy, J. Appl. Phy. 62 (1987) 2599). Because such cantilever chips can be produced by a batch process, the production cost is considerably reduced. Accordingly, at present, the cantilevers produced by utilizing the semiconductor IC manufacturing process have become mainstream of the SPM cantilevers.
A conventional method for producing an AFM cantilever made of silicon nitride film, utilizing the semiconductor IC manufacturing process, will be described with reference to FIGS. 12A to 12F (first prior art). First, as shown in FIG. 12A, a silicon nitride film pattern 902 is formed on the main surface (the (100) plane) of an Si substrate 901, thereby forming a starting substrate 900. Thereafter, as shown in FIG. 12B, the Si substrate 901 is subjected to anisotropic wet etching with KOH or the like, using the silicon nitride film pattern 902 as an etching-resistant mask. As a result, a pyramidal replica hole 903, serving as a mold of a probe of a cantilever, is formed on part of the main surface of the Si substrate 901. Then, as shown in FIG. 12C, the silicon nitride film pattern 902 is removed, and a new silicon nitride film 904, serving as a material of a cantilever, is deposited on the main surface of the Si substrate 901 including the exposed surface of the replica hole 903. Further, as shown in FIG. 12D, the silicon nitride film 904 is selectively etched into the shape of the cantilever, thereby forming a cantilever pattern 905. Then, as shown in FIG. 12E, Pyrex 906 to be a supporting member of the cantilever is anode-bonded to a predetermined region of the cantilever pattern 905. Subsequently, as shown in FIG. 12F, the Si substrate 901 is entirely removed by etching, thereby forming an AFM cantilever 907 having a supporting portion 906, a lever portion 905a and a probe portion 905b.
Thus, the AFM cantilever manufactured in the above method comprises a supporting portion made of glass, and probe and cantilever portions formed of silicon nitride film so as to be integral with each other. The AFM cantilevers having such a structure can be produced in a high reproducibility with a high accuracy of the micro n order. In addition, since the probe is made of silicon nitride film, which has a hydrophilic nature, it is applicable to AFM measurement in a liquid suitable to a living organism sample.
Recently, measurement with a scanning near-field optical microscope (SNOM) has attracted public attention. The SNOM has a resolution beyond the diffraction limit by virtue of an evanescent wave. Like the STM and the AFM, the SNOM is a kind of SPM, and uses a method for obtaining an optical image of a very high resolution by scanning an optically transmissible prove near a sample irradiated with light, utilizing the characteristic that the evanescent wave is located in a limited region smaller than the wavelength and is not transmitted in free space.
The measuring principle of the SNOM is to first cause the probe to approach near the surface of a measurement sample to a distance of about 1 wavelength or shorter, and prepare a map of intensity of light transmitted through a fine opening at the end of the probe, thereby resolving an image of the measurement sample. There ar e a number of measuring methods using the SNOM, which are divided broadly into two. One is called the collection method, in which an evanescent wave, transmitted through the sample when the sample is irradiated with light and located in a limited region near the surface of the sample, is detected through the probe to form an SNOM image. The other is called the emission method, in which the sample is irradiated with light emitted from the probe having a minute opening and light transmitted through the sample is detected by an optical detector arranged under the sample. This method is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-291310.
It is reported that the aforementioned AFM cantilever having a probe made of silicon nitride film can be applied to the SNOM measurement (N. F. van Hulst, M. H. P. Moers, O. F. J. Moordman, R. G. Tack, F. B. Segerink, B. Bolger; Near-field Optical Microscope Using a Silicon-nitride Probe, Appl. Phys. lett. 62,461 (1993)). Since silicon nitride film is capable of transmitting light, the AFM cantilever having a probe made of silicon nitride film is applicable to the SNOM measurement.
AFM measuring methods have a contact mode and a non-contact mode. The contact mode includes a method of measuring the surface of a sample in a state where the sample and the probe are in contact with each other and a tapping mode of measuring the surface of a sample, while tapping the sample surface. The non-contact mode includes a method of measuring the surface of a sample in a state where the sample and the probe are separated from each other at a distance of 5 to 10 nm and an oscillation mode (AC mode) in which the sample or the probe is oscillated at a constant frequency in a non-contact state. Of these methods, the AC mode and the tapping mode require a hard cantilever.
To produce a hard cantilever, a lever film must be thick. As an example of the hard cantilever, a type of AFM cantilever is widely known, in which the probe, the lever, and the supporting portion are integrally formed of silicon (e.g., O. Wolter, Th. Bayer, and J. Greschner; Micromachined Silicon Sensors for Scanning Force Microscopy, J. Vac. Sci. Technol. B9(2), May/April 1991) (second prior art).
The cantilever integrally formed of silicon is produced by the semiconductor manufacturing technique like the aforementioned AFM cantilever. Therefore, it can be produced in a high reproducibility with a high accuracy on the micron order. In addition, since a lever is constituted by a silicon substrate, a thick lever can be produced easily. Since the probe is made of silicon, the conductivity can be added to the probe by diffusing an impurity into silicon in advance. It is therefore possible to perform STM measurement and surface decoration or process by using the probe.
The SPM cantilever for use in, for example, an AFM, in which a probe and a lever are integrally formed of silicon (first prior art), has the following drawbacks. The thickness of the cantilever is determined by the thickness of silicon nitride film deposited after forming a replica hole. However, to avoid a crack or a bend of the substrate due to the stress in deposition of silicon nitride film, the thickness of the film which can be deposited is limited to about 1 .mu.m. It is therefore impossible to form a hard cantilever useful for measurement of the AC-mode or tapping mode. Further, since the probe is made of silicon nitride film, i.e., insulating film, it is difficult to apply the probe to the STM measurement and surface decoration or process.
Furthermore, in the cantilever made of silicon nitride film, to avoid a bend of the lever after removing the substrate by etching, the ratio of Si to N contained in silicon nitride film must be higher than 3:4 of the normal semiconductor IC. The inventors found that the optical transmittance is lowered in the silicon nitride film containing Si in a higher ratio, since light is absorbed by Si in a region of a short wavelength shorter than 400 nm.
As described before, the SPM cantilever formed integrally with silicon nitride is applicable to the SNOM measurement. To analyze by the SNOM a spectrum of fluorescence emitted from the measurement sample, the optical transmittance of the probe in a wide wavelength region is required. However, since the probe and the lever are formed integral with each other, the probe is made of the silicon nitride film containing Si in a high ratio, which lowers the optical transmittance in a short wavelength region. The probe is therefore unsuitable for such a measurement.
In the conventional SPM integrally formed as mentioned above, since both the probe and the lever are formed of silicon nitride, part of evanescent light input through the end of the probe or light scattered in the probe is transmitted from the probe to the lever. Therefore, the amount of light, which can be applied to the SNOM measuring optical detector (e.g., the photodetector) arranged above the cantilever, is as small as 100 pW. For this reason, the loss of the light detected by the optical detector is great relative to the light incident on the end of the probe, resulting in the problem that the SNOM measurement cannot be performed with a high sensitivity.
In the AFM cantilever in which the probe, the lever and the supporting member are integrally formed of Si (second prior art), since the probe is made of Si, in order to perform AFM measurement in water, which is available for living organism, it is necessary to coat the probe surface with hydrophilic film, thereby changing hydrophobic Si to hydrophilic. In this case, the sharpness of the end of the probe is reduced. Moreover, since the supporting member is formed of Si so as to be integral with the lever, it is necessary to form patterns on both surfaces of the substrate, and a special manufacturing apparatus is required.
Further, whether the cantilever has a probe made of silicon nitride or silicon, the mechanical strength thereof is not sufficient for the ultra-refined machining technique. As described above, in the AFM cantilever produced by the conventional semiconductor manufacturing technique, it is difficult to simultaneously satisfy the requirement for the probe and the requirement for the lever, since the probe and the lever are formed of the same material.